Quick answer
Water damage under a window comes from either rain getting in around the frame or condensation running off the glass. If it gets wet during storms and the stain starts at a bottom corner, it is a frame leak, and water is probably inside the wall. If it gets wet on cold mornings with beads on the glass, it is condensation. Press the drywall under both corners. Soft or swollen means the wall cavity needs to be measured and dried. Call (888) 840-6512 to reach a certified water damage restoration company in your area; the line is answered 24/7.
Written & reviewed by Marcus ReedIICRC WRT
Reviewed September 30, 2026· Next review Mar 2027
Water damage under a window shows up as a stain below the corner of the frame, paint bubbling on the wall, a soft sill, or drywall that crumbles at the bottom corner. It comes from one of two places: rain getting in around the window from outside, or condensation running down the glass from inside. They look similar and are fixed very differently, so the first job is telling them apart.
What to Do Now
- 1Dry what you can see and mark the edge. Wipe the sill and the wall, then pencil a line around the stain so you can tell if it grows.
- 2Work out whether it is rain or condensation. Note when it gets wet: during storms, or on cold mornings with water beads on the glass. The table below sorts it.
- 3Stop the water at its source. For condensation, lower indoor humidity and open the blinds. For rain, keep the window closed and shut, and cover the outside with plastic if a storm is coming.
- 4Check inside the wall below the window. Press the drywall under both corners and look at the baseboard. Soft, swollen or cool to the touch means water has gone into the wall cavity.
Rain Leak or Condensation?
| What you notice | Points to | Why |
|---|---|---|
| Wet during or after rain, dry otherwise | A leak around the frame | Failed caulk, flashing, or the sill pan |
| Wet on cold mornings, beads on the glass | Condensation | Warm indoor air meeting cold glass |
| Stain at a bottom corner, spreading down the wall | A leak around the frame | Water tracks down the frame to the lowest corner |
| Water between the two panes of glass | A failed window seal | Cosmetic; the wall is not usually involved |
| Wet on one side of the house only | A leak around the frame | The side that faces the weather |
Condensation mostly wets the sill and the glass, and it leaves mildew in the corners of the frame. The mold on window sills page covers that version. A frame leak is the one that gets into the wall.
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Why a Frame Leak Is Worse Than It Looks
Windows are installed with a flashing system that should send any water that gets past the frame back outside. When the caulk cracks or the flashing was never lapped properly, rain runs behind the siding, down the side of the rough opening, and into the wall under the window. The drywall stain appears late. By then the sheathing, the bottom plate and the insulation under the window have often been wet through several storms.
Wet wood and drywall start growing mold in about 24–48 hours, and the inside of a wall is dark and still. A stain under a window that has come back after being painted is a sign the water is still getting in.
Who Fixes What
There are two jobs. The window installer, a siding contractor or a general contractor stops the water: new flashing tape, a sill pan, sealant, or sometimes a reinstalled window. A restoration company deals with the wall. It measures the drywall and the framing below the window, opens the wall if the readings are high, dries the cavity, and removes wet insulation. Drying clean rainwater runs around $3–$7 per square foot before the wall is closed up.
The order matters. Repainting a stain before the leak is fixed and the wall is dry traps the water behind fresh paint.
If the drywall under your window is soft or the stain keeps growing after rain, (888) 840-6512 is answered 24/7 and connects you with a certified restoration company that serves your area. The company gives you an arrival window for your address on the call, and there is no charge for the referral.
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Frequently Asked Questions
Why is there water damage under my window?
Either rain is getting past the caulk, flashing or sill pan around the frame, or warm indoor air is condensing on the cold glass and running down onto the sill. Frame leaks get worse in storms; condensation shows up on cold mornings.
How do I know if my window is leaking into the wall?
Look for a stain that starts at a bottom corner and spreads down, paint bubbling under the window, and drywall that feels soft or cool. A stain that comes back after painting means water is still getting in.
Is condensation on windows a problem?
A little on cold mornings is normal. Heavy, daily condensation soaks the sill and frame, grows mildew in the corners and can rot a wood sill. Lowering indoor humidity is the fix.
Who fixes a leaking window?
A window installer, siding contractor or general contractor fixes the flashing, sealant or sill pan. A restoration company dries the wall under the window and removes wet insulation, after the leak is stopped.
Can water damage under a window cause mold?
YesWater from a frame leak runs into the wall cavity, where the sheathing, framing and insulation stay wet. Mold starts on wet material in about 24–48 hours.
Who do I call right now for water damage?
Call (888) 840-6512. The line is answered 24 hours a day, seven days a week, and connects you with an IICRC-certified water damage restoration company that serves your area. Describe what you can see — where the water is coming from, how long it has been there, and whether it smells — the more specific you are, the faster the right company is on the line.
If the source is a pipe or an appliance, shut the main water valve first. Have four things ready and the call goes quickly: the address, when the water started, what is wet now, and whether you have filed an insurance claim.
Still not sure which of these is your situation? (888) 840-6512 · toll-free · answered 24/7
Sources
- IICRC S500 Standard for Professional Water Damage Restoration
- EPA — A Brief Guide to Mold, Moisture and Your Home
- EPA — Moisture Control Guidance for Building Design, Construction and Maintenance
Methodology: How we source and verify data · Report an error
Disclaimer: HearthDry is an independent educational resource. This article is for informational purposes only and does not constitute professional, legal, or insurance advice. Consult licensed professionals before making decisions about your property or insurance claims.
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